Picked up LostMagic the other week. Interesting game. Your character is a squishy mage, and aside from basic elemental attack/healing magic (and regenerating MP) the main feature is capturing and commanding squads of monsters.
Anyway, I hate to say that the Fire Sage kicked my ass about half a dozen times last night. I'm non-elemental (but have every attack element at my disposal, natch), she's Fire, so that means she's weak to Water. (Traditional Fire/Water rivalry.) But since she's a squishy mage too, her Magic Defense is high and she's got access to the same powerful healing magic I do. If I go toe-to-toe with her I can nearly kill her but the moment I need to recharge my MP she just heals it all back. Grr!
So I went back to do some grinding (especially on my Water magic) and realized I was using the wrong strategy all along: Sure, you can't go wrong claiming all the Mana Crystals in the battle (faster MP regeneration, heals HP when nearby) but the mistake I made was trying to bring in a bunch of Water-type monsters to fight her with because those monsters don't actually have Water attacks on them, so their Water type is actually a liability against her powerful Fire spells.
So I swapped out my unit of Water wolves for a unit of Fire wolves and subsequently won the battle hands down. Sure, it was me taking out all the monsters around the perimeter, but after that I basically just selected my wolf unit, told them to sic her, and she goes down in a flurry of wolf bites.
Definitely have to keep the fight-fire-with-fire strategy in mind.